DIGITAL ELEVATION MODEL PROCESSING : FKKN AIRSPACE CASE STUDDY

The purpose of this Jupyter Notebook is to guides you through the process of Digital Elevation Model processing using open source tools. The case study focus on FKKN airspace, but the same process can be used for any other airspace.

1. BACKGROUND

What is Digital Elevation?

Digital Elevation Model (DEM) is simply a mathematical representation of the continuous surface of the ground, based on a (large) number of points defined in terms of X, Y and Z co-ordinates. Typically, DEM data can be represented as a raster which is most easily expressed as being a 2D array with each individual cell having an elevation associated with it.

What is a Raster?

Raster or “gridded” data are stored as a grid of values which are rendered on a map as pixels. Each pixel value represents an area on the Earth’s surface. A raster file is a composed of regular grid of cells, all of which are the same size. You’ve looked at and used rasters before if you’ve looked at photographs or imagery in a tool like Google Earth. However, the raster files that you will work with are different from photographs in that they are spatially referenced. Each pixel represents an area of land on the ground. That area is defined by the spatial resolution of the raster.

2. DEM APPLICATION IN AVIATION

Terrain and obstacles data have many applications in aviation insdustry:

3. FKKN AIRSPACE PRESENTATION

The structure of FKKN airspace is describe in AIS ASECNA. We have three main features:

3a Import python librairies

3b Set path to files

3c Plotting airspace

3d Plotting airspace with basemap

We use OpenStreetMap.Mapnik base map

4 Buffering FKKN TMA

The following statement creates an buffer airspace of 10 NM around the the FKKN TMA. We choose 1ONM buffuring because, we will use that portion of airspace later for Mountainous area calculation inside TMA FKKN in accordance with ICAO provisions in DOC DOC 8168 VOL II, PANS OBS

4a Buffer opeartion

Buffer visualisation

Let see the buffer we just create

4. DIGITAL ELEVATION MODEL PROCESSING

The DEM we will process in this section is a lager data set covering the entire Cameroon.

It is provided by Cameroon Civil Aviation Authority ATS Bureau.

The same DEM is used to design Instrument Approach Procedure of Ngaoundéré Aerodrome.

4a. DEM Exploration

4a1. Deep into the folder containing our DEM

We note that, the folder containing the entire DEM have 268 files representing 67 blocks of DEM.

The data format for each block is a Band Interleaved by Line: .BIL. The meta data are depicted in 3 differents files with extension:

4a2. Let explore your DEM in deep

Open a rondom block for data exploration

DEM coordinate reference sustem

DEM coordinate reference sustem as wel known text (wkt)

DEM Resolution

Number of Bands

The width

The height

The Driver (data format)

No data values for all channels

All Metadata for the whole raster dataset

4a3. Merge all small block in a single file

We saw previously, that our DEM is split into 67 blocks of .BIL file. In this section , we will merge all small blocks of DEM into a single larger Geotiff file cameroon.tif.

The challenge here is to do the operation without loosing any data or meta data information

4a4. Check to confirm that we did not lose information during the process

We note that all these information is the same that we got before processing. We did not loss information during the merging processs

4b. CLIP THE IMAGES

As we see previously, the DEM covers the entire cameroon, it means more than 474 444 square kilometers. We are highly motivated to reduce the size of an image to only our area of interest (AOI). The best way to accomplish this reduction is to clip an image to a boundary that defines our study area.

4b1. Cemeroon DEM visualisation

Let see how look the DEM that we want to clip.

4b2. Plot the your AOI inside the DEM

4b2. Extraction of our AOI (DEM OF TMA FKKN + 10NM BUFFER)

We extract our AOI in color : magenta on the image above

4b2. DEM OF TMA FKKN + 10NM BUFFER plotting

4b3. PLOT ALL TOGETHER

5. FKKN TMA + 10NM BUFFER DEM ANALYSIS

Now that we have extrated the DEM of our AOI, It is time to do some analysis and products derivations.

In this part, we will cover:

5a. Color ramp

When we apply a ramp of color to elevation, it gives us a quick overview of how our elevation data look like. It is easy to know where we have high/low terrain

5b. Hills shade

Shaded relief maps color elevation in such a way that it looks as if the terrain is cast in a low angle light, which creates bright spots and shadows. This aesthetic styling creates an almost photographic illusion, which is easy to grasp so that we can understand the variation in the terrain. It is important to note that this style is truly an illusion as the light is often physically inaccurate in terms of the solar angle, and the elevation is usually exaggerated to increase contrast.

5c. Elevation contours

A contour is an isoline along the same elevation in a dataset. Contours are usually stepped at intervals to create an intuitive way to represent elevation data, both visually and numerically, using a resource-efficient vector dataset. Now, let's look at another way to visualize the elevation better using contours.

The image above show the elevation contour for the entire AOI, to better understand what going on, let zoom in into a small area, to see how it look like

6. FINAL PRODUCT

As mentionned at the begining of this work, DEM is used by Instrument Flight Procédure Designer (IFPD) for many tasks including (MOC calculation, Mountainous Area calculation, MSA, MDA/H or DA/DH, Contingency procedures, Drift-Down Procedures, Emergency En-route Landing, etc.).

DEM model is also used as background for many aeronautical charts like (Instruments Approach Chart(IAC), Standard Arrivals (STARs), Standard Instruments Departure (SID), etc.)

The final result of our analysis, is a product that can be used to design an Instrument procecure of Ngaoundéré Airport. It is important to mention that, the raw/original data that we use, is the same that CCAA uses for the ongoing process of Ngaoundere procedure design.

For example, aeronautical charts provide by JEPPESEN are georeferenced(each pixel on the chart match a physical location on the ground). When use in the onboard application like foreflight, the DEM model on the background of chart can significantly inscrease pilot awareness with respect to terrain clearance in a real time.

6a. Digital Elevation Model with color ramp applied ready to be used

We appliy color ramp to elevation contour line to get a DEM that can be use as a background map for aeronautical chart

Let zoom in to see how it look like

Few examples

The picture below show how elevation contours line and color ramp are use as background for aeronautical chart

Example 1: FKKL STARs RWY31

Example 2: FKKL IAC RNAV(GNSS)-RWY 31

6. SUMMARY

In this Jupyter notebbok, from the description of FKKN TMA in the AIS ASECNA, we build a spatial representation of this airspace in shapefile format. Then we use that shapefile to create a buffer of 10NM around . From a lager Digital Elevation Model(DEM) covering the entire Cameroon, we use that buffer as a mask to extract the portion covering our Area of Interest(AOI).

After processing the DEM of our AOI, we get an Elevation Contours line and Color Ramp which can be used as background for aeronatical charts (IAC, STARs, SID, etc.).

We also apply Reproducible science technique to this work,so anyone can understand and replicate the steps of the analysis, applied to the same or even new data. We can use the next step to any other airpace and get the result.

7. Next release

The next release will cover Mountainous Area calculation for Instrument Flight Procedure Design (IFPD).

The Mountainous area. An area of changing terrain profile where the changes of terrain elevation exceed 900 m (3 000 ft) within a distance of 18.5 km (10.0 NM). DOC 8168 VOL II.

The work will consist of finding the points (lat/long) in WGS84 coordinate reference system where the difference between the highest elevation and the lower elevation within 10NM is 900m (3000ft) or above.

These points are important for MSA and MOC calculation, as we refer to Annexe 2,Chapter 5, alinea 5.1.2, Minimum Flight Altitude ( Except when necessary for take-off or landing, or except when specifically authorized by the appropriate authority) is 2000 ft over high terrain or in mountainous area and 1000 ft elsewhere, above the highest obstacle located within 8 km of the estimated position of the aircraft.

The intended output will be :